Bulk material handling vehicles, such as railway cars or trucks are generally provided with a discharge hopper having a selectively operable discharge door for controlling flow of the bulk materials through an opening at the bottom of the hopper. Such discharge doors are sometimes manually operated, but are frequently controlled by a power device such as an hydraulic cylinder. The doors often slide in guiding channels, and these channels tend to become clogged with the bulk materials so that it can be difficult to start a door opening, or move it through a small increment in either direction, especially when the bulk materials above the door are applying high gravitational pressures on it. The doors are sometimes mounted on rollers so as to make them easier to move, for example as shown in Sherman et al. 1,360,805, or Empey U.S. Pat. No. 4,004,700, such doors then being reciprocated back and forth across the opening by the piston rod of an hydraulic cylinder which is coupled by suitable coupling means to the door. The piston is often mounted offset longitudinally or transversely of the hopper so that it is not directly beneath its opening, some doors using a linkage mechanism as shown in Elliott U.S. Pat. No. 2,303,033 whose door is also supported on rollers.
It is also frequent practice to mount a rack along each longitudinal edge of the door and engage the rack with spaced pinions which are fixed to a common shaft so as to prevent the door from cocking and binding in the longitudinal channels which support it, such structures being shown in Allen et al. U.S. Pat. No. 1,384,175 and in Bessette U.S. Pat. No. 4,051,785.
Nevertheless, despite the use of power cylinders and antifriction supporting means, the doors do tend to stick, and are especially difficult to move through small increments, as may be required when the hopper is being used to discharge a bulk material at a measured rate, for instance when spreading particulates on a highway. A distinction must be drawn between on the one hand opening of a door for the purpose of completely discharging the hopper at a high discharge rate, as might be desirable when emptying the hopper into some other container, and on the other hand the concept of discharging the hopper contents at a very slow and carefully measured rate as would be required during spreading of the material over a large area where uniformity of discharge rate is a requirement. In the latter case, it is not only difficult to start the door moving using the hydraulic cylinder, especially when the hopper is full, but it is also difficult to move the door through a small closely predetermined increment as is required when spreading materials over a large area. The present invention seeks to provide improvements in these modes of operation, without unnecessarily increasing the cost of the device.
Copies of the five patents discussed above are filed with this specification in lieu of a prior art statement.